A typical expansion engine used in cryogenic refrigeration is shown in U.S. Pat. No. 4,466,251 to Chellis and Lessard. In such refrigerators, a piston is reciprocated within a cylinder. One end of this cylinder has a cold end and is placed within a cold insulated environment. Cold, high pressure gas such as helium is introduced into the cold end of the cylinder by a first valve. With upward movement of the piston, that cold gas is further cooled by expansion, and with downward movement of the piston the gas is then exhausted through a second valve. From this second value, exhausted gas is returned to ambient temperature through a heat exchanger located in the cold insulated environment. The gas passes through the heat exchanger in a counter flow heat exchange relationship with the incoming high pressure gas and, thus, cools that gas. With such an arrangement, the high pressure and the exhaust valves are positioned in the cold environment.
In order to activate both the inlet and the exhaust valves, a fluid activated control valve is positioned within the cold region of the refrigerator. As control fluid flows to the valve that fluid is cooled by a thermal regenerator. When the control fluid is exhausted, heat is restored to the fluid as it passes through the regenerator. Therefore, heat is not transferred to the high pressure or exhaust valves by the control fluid.
Currently, the activated control valve is a stepped cylinder/piston combination with very small radial clearances (e.g., 0.0001 inch). Presently, in manufacturing this step combination, the cylinder must be made of two pieces with a lapped through-hole in each. The pieces are then joined with a pliable vacuum tight seal at the parting line whereby a step shaped, concentric cylinder is formed. The problem with this assembly is that under high pressures (approximately 300 PSI) and low temperatures (approximately 4 K) this seal becomes difficult to make reliably. Other attempts such as aligning two bores by a milling machine so that the small radial gap between the piston and the cylinder is maintained proved to be too impractical in manufacturing processes.
An improvement is, therefore, needed for a more reliable means of manufacturing a stepped cylinder/piston combination with very small radial clearances.